1. Field of the Invention
The present invention relates to a light-polymerizable resin composition for making the model or the pattern of a clasp showing a theoretical configuration for providing a retaining force, durability and so on, which is designed to be used to prepare a cast clasp by the lost wax process, said cast clasp being a metallic component or part for gripping (bearing the resistance to lateral movement), retaining (bearing the resistance to attachment or detachment) and supporting (bearing the resistance to an occlusal load) a partial denture, when said partial denture is prepared.
2. Statement of the Prior Art
More than 90% of the direct or indirect retainer used for partial dentures have been clasps. However, not until now has the cast clasp in particular been satisfactory in view of its retaining force, fitness, the form of a given arm etc., since it has been troublesome to prepare.
Referring to the conventional preparation of cast clasps, a gnathic impression is first taken out of the patient's mouth, and a working model is then formed with super-hard gypsum, etc. Subsequently, a tooth portion retained on the working model is surveyed so as to design a clasp that will become a retainer. It is noted that surverying is a work for finding teeth remaining on the working model or the maximum protuberance of an alveolar ridge and, at the same time, to examine relative parallelism, etc., and is very important for the preparation of dentures in view of the purposes of the work which include the determination of the direction of denture attachment/detachment, the determination of the tooth to be retained, the indication of the region to be blocked out, the measurement of the amount to be undercut and the determination of the position of a clasp.
The important factors that govern the retaining force of the clasp include, by way of example, the length, thickness and sectional shape of a clasp arm, the Young's modulus of the alloy used, the present amount of the undercut and the curvature of the surface of the tooth retained. When preparing a cast clasp making use of a paraffinic wax pattern, the retaining force of the clasp may be controlled by the selection of the available amount to be undercut and the thickness and varied sectional shape of the pattern, and the clasp should be designed corresponding to the parodontium state of the tooth retained at the time of surveying or denture designing.
When design and blocking-out is completed for a working model, then the retained tooth portion blocked out according to the design for making the wax pattern of the clasp is again subjected to impression taking (double impression taking) to substitute a modeling investment for the working model. Then, while taking into account the precise control of the retaining force of the clasp, the clasp pattern of paraffinic wax is bent by fingers, and is put on the modeling investment with a bonding agent to form the rest and leg portions and provide a sprue. Next, the modeling investment is entirely invested in a casting investment without removing therefrom the clasp pattern of paraffinic base wax, followed by curing of the investment. Afterwards, the clasp pattern of paraffinic wax is heated and incinerated in a furnace. A molten metal is cast in the resulting die of the clasp pattern, and is cooled and subjected to indexing polishing to complete the clasp. Hereinafter, this method will be referred to as the entire-model-investing method.
The aforesaid method for cast clasp making involves the following problems, since paraffinic wax is used as the material for clasp pattern making.
(1) Paraffinic wax is entirely free from elastic deformation and undergoes plastic deformation. It is thus impossible to remove it from the undercut of the tooth retained on the working model through the maximum protuberance thereof.
Forcible removal of the wax causes it to deform so permanently that any accurate fit is not achievable. For that reason, it is impossible to make any precise clasp having a controlled retaining force. Required to this end is the so-called entire-model-investing method; it is essentially required that the working model be subjected to double impression taking to substitute a modeling investment therefor, and that a clasp pattern be formed on the modeling investment, which is then entirely invested in a casting investment. This method is troublesome and timeconsuming and gives rise to an increase in the material cost.
(2) An exclusive bonding agent has to be always used for the clasp pattern of paraffinic wax, since it shows no adhesion to gypsum. Thus, it is impossible to obtain any clasp showing a good fit, since the resulting clasp exhibits an error of about 50 .mu.m in the thickness of the adhesive layer.
(3) The clasp pattern of paraffinic wax is pressed and bonded to the modeling investment by fingers with the use of a bonding agent. At that time, however, the particles of the investment peel off due to its low strength, so that the form and surface nature of the tooth retained degrades. The peeling-off investment particles are also deposited onto the clasp pattern of paraffinic wax, thus making it impossible to obtain any cast clasp excelling in surface nature.
(4) The clasp pattern of paraffinic wax is a polymer having a high crystallization and a low molecular weight, and is of a substance which has a glass transition temperature Tg very close to a melting point Tm, but has no glass transistion point as is the case with a metal. Hence, when the wax is incinerated by heating, too high a heating rate causes the molten wax to boil and vaporize into a gas, which in turn expands, and rapidly pressurize the investment with the result that it may crack or roughens on its inner surface.